As described in copending patent application Ser. No. 08/413,555 filed Mar. 30, 1995 to Guo Ping Li et al., entitled "Multi-wavelength gain coupled distributed feedback laser array with fine tunability", semiconductor lasers fabricated from group III-V alloys, can be tailored to emit radiation in the wavelength range from 1250 nm to 1600 nm for wavelength division multiplexing (WDM) communications technology.
While it is known to select discrete laser devices having a specific wavelength and to assemble hybrid arrays of such devices, pre-selection to locate lasers of specific wavelengths, and subsequent assembly and packaging are time consuming and costly. Reliability issues may give rise to variable performance with aging.
It has been proposed to thermally tune the central wavelength of individual semiconductor lasers by using integrated heater, as described for example, by L. A. Wang et al. in "Integrated four wavelength DFB laser array with 10 Gb/s speed and 5 nm continuous tuning range" in I.E.E.E Photonics Technology Letters 4(4) pp. 318-320 (1992) and by Sakano et al in "Tunable DFB laser with a striped thin film heater" in I.E.E.E Photonics Technology Letters 4(4) pp. 321-323 (1992) and in U.S. Pat. No. 5,173,909 issued Dec. 22, 1992, entitled "Wavelength Tunable Diode Laser". These references describe how tuning of 4 or 5 nm can be achieved by monolithically integrated heaters, i.e. resistors, to provide a laser array with multi-wavelength capability. Heating may induce band gap shrinking in semiconductors and change the crystal refractive index. Nevertheless, an extended range of tuning is required to provide a useful array with multi-wavelength capability, as explained by Guo Ping Li et al. in the above mentioned reference. Moreover, precise temperature control is required to prevent overheating, which may cause power reduction and reduce quantum efficiency.
Existing thin film resistors used as resistive heaters for tunable laser arrays are typically formed from a resistive layer of Ti with Au/Pt/Ti contact pads. These resistors are often insufficiently reliable, and sensitive to atmospheric humidity. Application of thermal fine tuning of laser diode arrays using Ti thin film resistors is not sufficiently reliable for practical applications.
Other resistance layers commonly used are NiCr, or Pd. Difficulties have been encountered in obtaining a satisfactory combination of low resistance, good controllability, and stability with known thin film resistors fabricated from these materials.